In many circuits, the power that is available to drive the circuit may not be in a form that the circuit demands. To correct this, it is useful to provide a power converter that converts the available power into a form that conforms to the circuit's requirements.
One common type of power converter is a switch-mode power converter. A switch-mode power converter produces a voltage by switching reactive circuit elements into different electrical configurations using a switch network. A switched-capacitor power converter is a type of switch-mode power converter that primarily utilizes capacitors to transfer energy. Such converters are called “charge pumps.” The capacitors are called “pump capacitors.”
In operation, a charge pump transitions from one pump-state to the next in a sequence of pump-states. Each pump-state is characterized by a residence time in which the charge pump remains in that pump-state, and transition times, in which the charge pump is between pump-states. The sum of the residence times for all pump-states and the intervening transition times between those pump-states is the period for one cycle of the charge pump.
For correct operation, each pump capacitor should begin and end each cycle with zero change in charge. If this is not the case, charge will accumulate on the pump capacitor over the course of several cycles in the case of positive non-zero change in charge. Since the voltage across a capacitor is linearly proportional to the charge, this charge accretion/depletion will cause the voltage across the pump capacitor to drift over time.
In many charge pumps, a switch connects adjacent pump capacitors. The voltage across the switch thus depends on the voltages across adjacent pump capacitors. If voltages across these capacitors drift unevenly, the voltage across the switch may exceed its rating. This may cause the switch to overheat, thus destroying the switch, and the charge pump as well.
Procedures for managing charge on a pump capacitor depend in part on how the charge got there. In general, there are two ways to put charge into a capacitor: using a voltage source or using a current source.
When a voltage source is used, management of charge is relatively simple. The charge present at a capacitor is a linear function of the voltage. Thus, dropping the voltage to zero is sufficient to remove the charge from the capacitor.
When a current source is used, management of charge is not so simple. This is because the charge on a pump capacitor is related to an integral of the current, and not to the instantaneous value of current.
On Nov. 8, 2012, Patent Publication WO 2012/151466, which is incorporated herein by reference, made public configurations of charge pumps in which one terminal was connected to a regulator. Because of its inductor, and because of the relevant time scales associated with the switches involved, as far as these charge pump configurations are concerned, the regulator behaved like a current source. This made management of how much charge is in the pump capacitors more challenging.